1. Field
The embodiments discussed herein are directed to a moving image coding apparatus and a moving image coding method for intra-coding particular images and inter-coding the other images.
2. Description of the Related Art
Moving image data generally has an extremely large data amount. In upon an apparatus dealing with the moving image data attempts to send the moving image data to another apparatus or store the moving image data in a storage apparatus, the apparatus compresses the moving image data. Coding methods employed for compressing the moving image data include an intra-coding method and an inter-coding method. The intra-coding method is a method of using only information included in one picture of a coding target and coding the relevant picture. This method is also referred to as intra-frame coding method or intra-picture coding method. On the other hand, the inter-coding method is a method of using information on a picture of a coding target and information of previous and subsequent pictures and coding the picture of the coding target According to the inter-coding method, the picture of the coding target is divided into a plurality of blocks. With respect to the respective blocks, from pictures obtained by decoding already coded previous and subsequent pictures, a most similar area is selected, and difference information between the respective blocks and the selected similar area is obtained. The difference information and motion vector information representing amounts at which the respective blocks are moved from the similar area are coded. For that reason, according to the inter-coding method, as redundant information can be removed, it is possible to achieve a compression efficiency higher than a compression efficiency based on the intra-coding method.
In contrast to this, according to the intra-coding method, in general, it is possible to reduce an error between the decoded picture obtained by decoding the coded data and the original picture. For that reason, an apparatus for reproducing the compressed coded moving image data can decode a picture with a high sharpness from the coded data according to the intra-coding method. On the other hand, according to the inter-coding method, as described above, only the difference information between the previous and subsequent pictures and the motion vector information are coded. If an error is included when any one of pictures is coded, the error propagates in other pictures after the relevant picture. In view of the above, in order to suppress the propagation of the coding error, a coding method of carrying out the intra-coding at a constant frequency is proposed. Such a coding method is, for example, adopted in standards such as ISO (International Organization for Standardization) Moving Picture Experts Group phase 2 (MPEG-2), MPEG-4, or ITU-T (International Telecommunication Union Telecommunication Standardization Sector) H.264/MPEG-4 Advanced Video Coding (H.264/MPEG-4 AVC). It should be noted that with regard to a plurality of continuous pictures repeated at this constant frequency, a coding method for the respective pictures is defined as Group Of Pictures (GOP). Also, hereinafter, among the pictures included in the GOP, a picture to be intra-coded is referred to as I-picture. Furthermore, a picture to be inter-coded by using the information on the temporally previous picture is referred to as P-picture. Furthermore, a picture to be inter-coded by using both the information on the temporally previous picture and the information on the temporally subsequent picture is referred to as B-picture. It should be noted that the picture is a frame or a field. The frame is one still image in the moving image data, and on the other hand, the field is a still image obtained by taking out only data in an odd-numbered row or data in an even-numbered row from the frame. A field having only the data in an odd-numbered row and a field having only the data in an even-numbered row are respectively referred to as top field and bottom field in the present specification.
A Constant Bit Rate (CBR) coding method is adopted in a system such as a digital broadcasting system having a high demand with respect to a real-time feature with regard to a moving image data distribution. This CBR coding method is a coding method of allocating a constant bit length to the coded moving image data for every unit of time. At this time, the I-picture located at the lead-in of the GOP functions as a reference for inter-coding other P-picture or B-picture. However, the compression efficiently of the intra-coding method is lower than the compression efficiently of the inter-coding method. In order to reduce the coding error of the I-picture as much as possible, it is necessary to allocate the I-picture with a larger information amount, that is, a larger number of coding bits. If the I-picture is allocated with many coding bits, as the number of bits per unit of time is decided in the CBR coding method, the number of coding bits which can be allocated to the P-picture and the B-picture in the GOP is smaller.
The system having the high demand with respect to the real-time feature suppresses a delay due to the coding of the moving image data, and the B-picture requiring the information on the previous and subsequent picture for the coding is not used in many cases. If the B-picture having the highest compression efficiency is not included in the GOP, the number of coding bits allocated to the P-picture is inevitably insufficient. As a result, the sharpness of the P-picture decoded from the moving image data which is coded through this CBR coding method is decreased. On the other hand, as described above, the I-picture is allocated with the sufficient number of coding bits, and thus the decoded I-picture has a high sharpness. Due to this difference in the sharpness between the P-picture and the I-picture, a subjective image quality of a viewer with respect to the decoded moving image data is decreased.
With respect to such a problem, it is reported in the following paper that if a high image quality picture and a low image quality picture are alternately arranged in the moving image data, the subjective image quality of the viewer with respect to the moving image data is satisfactory.
FUKINUKI, “Sharp/Blurred Sampled Motion Pictures and Its Application to Picture Coding”
—Extension of the Signal Processing of Motion Visual Perception—
PCSJ2007/IMPS2007, No. P 3-03 (October 2007)
However, in the GOP dealing with a scene with a motion, a picture temporally closer to the picture of interest has more similarity. Therefore, according to the inter-coding method, as a reference picture functioning as a reference for the inter-coding, by using the temporally closest picture, the coding error can be reduced. However, as described above, upon the moving image data is inter-coded so that the high quality pictures and the low quality pictures are alternately arranged from the lead-in of the GOP, the picture temporally closest to the picture of interest may have a substantially degraded image quality due to the coding. That is, it is conceivable that as the coding is carried out Based on the degraded image, the coding errors may be accumulated. The P-picture included in a second half of the GOP has the prominent image degradation due to the accumulation of the coding errors, and therefore a difference in image quality between the P-picture included in the second half of the GOP and the image quality of the I-picture immediately after the P-picture is extremely large. In the decoded moving image data, flicker noise generated per unit of GOP is conspicuous.